The HHG can be well understood with the classical [1]} and quantum [2]} three-step models.
These models describe the HHG as a three-step process:
the valence electron of atoms or molecules is ionized by the laser field; the freed electron propagates in the external field;
the electron is driven by the laser field to return to and recombine with the parent ions with the emission of a high energy harmonic
along the direction parallel to the laser polarization (parallel harmonic).
The quantum model [2]}, frequently termed as strong-field approximation (SFA), also predicts the emission of harmonics
along the direction perpendicular to the laser polarization (perpendicular harmonic). For a linearly-polarized laser field and for linear symmetric molecules,
the perpendicular harmonics appear only for molecular targets aligned along a direction not parallel or perpendicular to the laser polarization [4]}.
According to the SFA, parallel and perpendicular harmonics are emitted at the same instant and have no phase differences between them.
So the whole harmonics emitted are also linearly polarized.
